1. Field of the Invention
The present invention relates to a process and apparatus for sterilizing fluent materials without disturbing the natural flavor and stability of these materials. The invention has particular application to fluent food products.
2. Description of the Prior Art
The science of preserving food products has been studied continuously from the stone age to the present day. The application of heat to food products as a preservation technique was probably first used to cook and dry meat in stone age human societies. A major step forward in heat treatment of fluid food products was made in the 19th century with the development of pasteurization, a process of partial sterilization involving subjecting a substance, particularly a liquid, to a temperature for a period of time that destroys disease-causing organisms without major chemical alteration of the substance. Numerous other techniques have been developed more recently wherein fluent food products are completely sterilized to eliminate bacterial spoilage and permit storage without refrigeration. However, the affluent consumers of modern food products do not view the use of preserved foods simply as a technique of staving off starvation, but rather have the option to choose the most appealing food products at will. Thus the factors that make food products appealing to modern consumers have become the most critical factors to be observed in food processing and preservation. The most crucial of these factors without doubt are taste and convenience. Of these two factors taste is perhaps paramount although convenience is becoming more and more critical, especially as it relates to energy conservation.
Referring first to the concept of taste, it is believed clear that if modern consumers are presented with two preserved food products, one of which tastes the way they expect it to taste, and the other of which tastes even slightly different from the way they expect it to taste, the first product will attain wide commercial success, while the second product will be saleable, if at all, only if its cost is significantly less than that of the first product. Proper taste is an especially critical factor in products such as milk to which virtually everyone is exposed during his lifetime. Nearly every consumer has tasted milk and knows exactly how it should taste. In many cases consumers have also tasted sour or slightly sour milk and various forms of fully-sterilized or processed milk. Due to such wide-spread and often life-long experience, consumers develop an acute sensitivity to taste variation in milk products. Similar circumstances apply, although to a lesser degree, to other common products such as orange juice, beer, selected types of soup and the like, although milk as a product that one experiences virtually from birth, is a matter of particular sensitivity to consumers. Thus a major technical problem that has nagged the dairy industry from its inception is the development of a technique for fully sterilizing milk without perceptibly changing its taste. Although the industry has actively researched this problem since before the beginning of the twentieth century, every solution which has been proposed has failed due to the complex nature of milk itself and due to the high sensitivity of the consuming public to slight variations in the taste of sterilized or processed milk.
The second previously mentioned factor of major concern to moder consumers is that of convenience. Consumers are willing to pay a substantial premium for food products which are particularly convenient to use or store. Consumers are particularly well aware of the short shelf life of milk and the need to keep it refrigerated. Accordingly they customarily make far more trips to market for the purpose of purchasing milk than for any other reason, according to studies conducted by the U.S. Department of Energy. Such additional trips to market for the purpose of purchasing only one product are becoming more and more burdensome and inconvenient with the increasing cost and decreasing availability of automotive fuels. Furthermore, the need to continuously refrigerate fresh milk creates additional inconvenience and loss of economy. Refrigeration requires substantial energy and milk, normally being a bulky product, consumes a large portion of personal and commercial refrigerator space. The cost of fresh milk is raised by the extensive refrigeration energy expended by dairy producers, wholesalers and retailers of the product. Accordingly fresh milk as it is presently known and utilized is a product that creates considerable inconvenience in requiring numerous otherwise unnecessary trips by consumers to retail establishments and by the fact that continuous refrigeration is required. Both of these undesirable factors could be eliminated if fully sterilized milk were available. Such a product would have an extensive shelf life and would not require refrigeration so that consumers could purchase large quantities of sterilized milk at regular intervals for storage without refrigeration. Similarly, wholesalers and retailers could also store large quantities of the product without refrigeration, thereby reducing the overall cost of the material to the consumer. Consumers would experience multiple savings in utilizing a sterilized milk product in the form of fewer trips to the store resulting in less fuel consumption and, in some cases, the need for fewer automobiles per household as well as in possibly reduced energy costs due to the possibility of utilizing smaller refrigerators.
While sterilized milk clearly possesses a number of advantages from the point of view of convenience and energy saving, the problem of its production without substantial taste distortion relative to fresh milk has prevented sterilized milk from gaining a substantial foothole in the consumer market. It is the complex chemistry of milk which makes it particularly subject to changes in taste upon heat treatment. To fully understand this taste sensitivity of milk to heat treatment, it is believed that a brief summary of milk chemistry is in order.
It is well known to those skilled in the art that milk contains among its various constituents the following nutrient items:
Water PA1 Proteins, such as casein, lactalbumin, lactoglobulin PA1 Vitamins PA1 Gases PA1 Milk fat PA1 Lactose (sugar of milk) PA1 Milk ash PA1 Pigments PA1 Enzymes PA1 Cellular material
Each of these nutrients reacts differently upon exposure to various temperature ranges for selected time intervals. Thus any heat treatment of milk must take into effect the characteristics of these nutrients as well as other organisms such as bacteria, spores, yeast and mold present in non-sterile milk. Unfortunately, all of the relationships between the various elements constituting milk are not fully understood, even by those highly skilled in the art of milk chemistry. Thus it is only by experimentation with new techniques for sterilizing milk that a process and apparatus can be developed wherein sterilized milk is produced but still retains all of the desirable qualities and characteristics of fresh milk such as flavor, stability, body and color.
As a result of extensive experimentation, Elmer S. Davies and Frank D. Petersen developed a series of time-temperature relationships and a general technique which appeared promising in the development of sterile milk which maintains all of the desirable qualities of fresh milk. This development is disclosed in U.S. Pat. No. 2,899,320 (Davies et al), issued Aug. 11, 1959. As is pointed out in this basic patent, to be truly effective in producing a sterilized milk that retains all of the desirable characteristics of fresh milk, a considerable number of independent reactions must be either accomplished or avoided simultaneously. Specifically, living organisms must be completely sterilized and enzymes inactivated. However "browning" and coagulation must be avoided. "Browning" is due to the heat sensitivity of lactose and casein as present together in milk. Similarly, coagulation is a function of temperature resulting from the combination of casein, milk sugar and whey in the protein content of milk. Coagulation results in an undesirable increase in the viscosity of the milk and produces an "off" flavor which is highly objectionable and easily detectable by consumers. Furthermore, the release of sulfhydrils in the course of the heating process produces a "cooked" flavor in heated milk. Sulfhydrils are formed by the breakdown of the whey portion of milk proteins, particularly the beta lactoglobulin upon heat treatment of milk.
In the above-referenced Davies et al patent, the following time-temperature relationship was established as the most effective in attaining full sterilization of milk with minimum effect upon its desirable characteristics: heating to approximately 300.degree. F. for 1.5 to 3.0 seconds. Further experimentation has shown this relationship to have a temperature range of approximately 280.degree.-310.degree. F. and a time range of approximately 1.5 to 9.0 seconds. While this time-temperature relationship still remains optimum, it has since been discovered that more subtle factors are involved in maintaining the flavor of sterilized milk sufficiently close to that of fresh milk that consumers cannot detect the difference. These factors involve the extent of physical agitation or perturbation experienced by the milk during heating, the uniformity of heating and the extent to which the heated milk contacts surfaces hotter than itself during or subsequent to the heating interval. Furthermore, proper cooling and handling of the milk prior to and subsequent to heating have also been found to be a factor in maintaining taste perfection in sterilized milk. Because of the interaction of these many factors during the sterilizing process, extraordinarily sophisticated constraints have been imposed upon any sterilization system which is to be successful in maintaining perfect flavor quality during sterilization. The lack of detailed knowledge as to the effects of these very subtle factors on milk flavor has severely impeded the development in the prior art of any successful device or method which could successfully produce sterilized milk having a taste acceptable to the highly refined standards of modern consumers. Furthermore, the unique combination of constraints, once known, has substantially defeated prior engineers and researchers in their efforts to construct a truly satisfactory milk sterilizing apparatus.
Experimental studies conducted by Elmer S. Davies and Frank D. Petersen (see Davies et al) led to the conclusion that the risk of denaturation of milk proteins was reduced if sterilization was conducted at higher temperatures than previously used, but for shorter time intervals. The concept of heating milk to a high temperature for a short time led to further studies to determine how such heating could be most advantageously accomplished. It was eventually determined that a falling film of product provided the optimum configuration for attaining high temperature/short-time heating in view of the physical characteristics of a film. In particular, a falling film is ideally suited to rapid heating of a product because it is by nature a thin distribution of the product with a high ratio of heat transfer surface area to volume and optimum heat transfer characteristics. Thus all particles of the film can be rapidly heated to the desired temperature with excellent uniformity. The fact that the film is a continuous body of liquid enhances the uniformity of heating relative to a spray or other arrangement where droplets are separated and travel for different times along various trajectories which cannot be fully controlled. The falling film concept was also ideally suited to short heating times because the exposure time of the product to the heated environment can be very accurately controlled simply by controlling the height of the falling film. For these reasons Davies and Petersen selected a falling film as the product configuration optimally suited to the optimal time-temperature configuration they had experimentally determined for minimizing the denaturation of proteins in sterilized milk. Unfortunately, the successful formation and continuous maintenance of a falling film proved to be an extremely difficult technical problem which Davies and Petersen were unable to solve. The disclosure in the above-referenced Davies et al patent sets forth their proposed technique of providing a film which adheres by surface tension to guide plates, and is heated while in contact with these guide plates. For reasons which are made clear elsewhere in the present specification, heating a falling film while it is in contact with a guide plate of this nature is not suitable from a practical standpoint because flavor distortion occurs and the product burns onto the guide plate after a short period of use. Nevertheless the discovery that a falling film of product is ideally suited to the time-temperature relationship developed in the Davies et al patent remains an important advance in the state of the art of milk sterilization.
Of the prior art devices, the most advanced for producing sterilized milk that maintains taste qualities similar to that of fresh milk is disclosed in U.S. Pat. No. 3,771,434 to Davies, issued Nov. 13, 1973. The present invention is an improvement and an outgrowth of the apparatus disclosed and claimed in that patent. The apparatus disclosed in Davies relies upon a falling film of liquid milk which is guided by contact with a length of screen, wherein the falling film is subjected to high temperature steam for a short interval to cause sterilization. A number of important refinements have now been discovered which substantially improve its performance. More specifically, experimentation with the system disclosed in Davies has revealed that product taste, quality and long-term consistency could be significantly improved with proper modification of the disclosed system. It should be noted that the device disclosed in the Davies patent is far different from devices which have been relied upon in the past for evaporation of liquids. A device used for evaporation is disclosed, for example, in the Monsanto U.S. Pat. No. 441,106 issued on Nov. 18, 1890. In that patent a liquid is divided into fine droplets and subjected to heating whereby rapid evaporation of the falling liquid droplets occurs. Naturally, the use of such a system would be disastrous in the production of sterilized liquid milk because the evaporation which would occur, even if it were only partial, would significantly change the consistency of the milk, thereby making it highly undesirable to consumers.
A need therefore exists for an improved sterilization system for fluid or liquid foods wherein complete sterilization is obtained without adversely effecting the taste or other qualities of the food product.